The growth of the World Wide Web has led to a greater quantity and variety of information available to users online. Because much web content is made available for free or at relatively low cost to users, web site and web service providers often employ online advertising to generate revenue. In general, online advertising provides a means for displaying visual and audio ads to potential customers as they view web pages, perform searches through web-based search engines, use social networking sites, use electronic mail services, play games, view entertainment content, and generally engage in online activities.
One advantage of online advertising over more traditional forms of advertising (e.g., radio, television, and print ads) is the greater flexibility of online advertisers to tailor advertising campaigns based on time, location, context, and user segment (e.g. a group of users). In particular, online advertisers employ behavioral targeting to increase the effectiveness of an advertising campaign by targeting user groups with specific ads based on information that is known about the users' aggregate behavior and interests. However, because traditional behavioral targeting methods generally rely on pre-defined user segments that may be too broad or too narrow for a particular advertiser's needs, advertisers may miss valuable opportunities to provide the right advertisement that is more specifically targeted to a particular user segment or individual user.